Donald Trump stepped up his attacks against Hillary Clinton, seeking to exploit the FBI decision to reopen an investigation into her emails as America's bruising election campaign heads into its final stretch.

Just 10 days before the country goes to the polls to elect either the former US secretary of state or the bombastic Republican billionaire, America's top cop James Comey has been thrust center stage.

The FBI director wrote to lawmakers on Friday, announcing that his agents are investigating a newly discovered trove of emails, renewing an enquiry that the Clinton campaign thought ended in July.

Trump pounced. Campaigning in the western state of Colorado, which has been leaning toward Clinton, he denounced what he called his opponent's "criminal and illegal conduct," to chants of "Lock her up!"

"This is the biggest political scandal since Watergate, and it's everybody's deepest hope that justice at last will be beautifully delivered," Trump, 70, told a later rally in Phoenix, Arizona.

While his 69-year-old opponent remains on course to be voted in as America's first female president at the ballot box on November 8, her campaign is furious that its momentum has slowed in the final straight.

Clinton campaigned hard in the key battleground state of Florida on Saturday, greeting thousands of supporters at a Jennifer Lopez concert in Miami after earlier demanding that Comey explain in detail why he had effectively reopened the inquiry declared complete in July.

"Hillary Clinton's corruption is corrosive to the soul of our nation, and it must be stopped," the real estate tycoon said in Arizona.

On Saturday he also received the public endorsement of the father of 26-year-old aid worker, Kayla Mueller, who was kidnapped in Syria in August 2013 and killed in a 2015 coalition air strike.

Across the country at the Lopez concert, Clinton embraced the hit singer on stage and accused her opponent of stoking fear, disgracing American democracy and insulting "one group of Americans after another.

"Are we going to let Donald Trump get away with that? You're right. We're not," she said. "No matter what they throw at us, we don't back down. Not now. Not ever," the Democrat said.

- Trump vindicated? -

But Clinton's campaign has been overshadowed from the start by allegations she put US secrets at risk by using a private server based in her home for all email correspondence as secretary of state.

According to the New York Times, the probe was renewed after agents seized a laptop used by Clinton's closest aide, Huma Abedin, and her now estranged husband, Anthony Weiner.

Weiner, a disgraced former congressman who resigned in 2011 after sending explicit online messages, is under investigation over allegations he sent sexual overtures to a 15-year-old girl.

In July, Comey criticized Clinton's handling of sensitive information but recommended no charges be brought. Clinton appeared to be in clear.

Trump was outraged, using it as an argument that the White House race has been "rigged" against him by a corrupt elite.